Packers, bridge plugs, frac plugs, and other downhole tools may be deployed into a wellbore and set in place. Generally, such setting is accomplished using a system of slips and seals received around a mandrel. A setting tool is used to axially compress the slips and sealing elements, and thereby radially expand them. The slips, which often have teeth, grit, buttons, or other marking structures, ride up the inclined surface of a cone during such compression, and are thus forced outwards into engagement with a surrounding tubular (e.g., a casing or the wellbore wall itself). This causes the slips to bite into the surrounding tubular, thereby holding the downhole tool in place. The seal is simultaneously expanded by such axial compression into engagement with the surrounding tubular, so as to isolate fluid communication axially across the tool.
The seals are typically elastomeric, and have a tendency to extrude during setting and/or when a large pressure differential across the seals is present, such as during hydraulic fracturing. In particular, the seals may extrude through a gap between circumferentially-adjacent slips, which forms when the slips are expanded radially outwards. To address this tendency, backup members are sometimes positioned axially between the slips and the seals to block these gaps and prevent extrusion.